The invention relates to an MR method for forming MR images of an examination zone of an object to be examined, in which method MR data is acquired from the examination zone by means of a receiving coil system which includes at least one movable and/or flexible receiving coil, and in which the MR images are reconstructed from the MR data acquired. The invention also relates to an MR method of forming MR images of an examination zone of an object to be examined, in which method the examination zone to be imaged is excited by means of an excitation coil system which includes at least one movable and/or flexible excitation coil. The invention also relates to corresponding MR devices provided with a receiving coil system and an excitation coil system and means for the acquisition of information concerning the position and orientation of the coils. Finally, the invention also relates to a computer program for carrying out the MR method and/or for controlling the MR devices.
Magnetic resonance tomography (MR tomography) often utilizes receiving coil systems with a plurality of receiving coils instead of a single receiving coil in order to realize a better signal-to-noise ratio for a given MR sequence and a given acquisition time, and also an enhanced spatial resolution. Suitable reconstruction algorithms are required for this purpose; a number of such algorithms is known. A method which allows for a significant improvement to be achieved is the so-called SENSE method which is known from xe2x80x9cSENSE: Sensitivity Encoding for Fast MRIxe2x80x9d, Pruessmann, K. et al., Magnetic Resonance in Medicine, 42:952-962 (1999). The SENSE method mainly enables a reduction of the measuring time by means of a practical method. Further methods mainly aim to enhance the signal-to-noise ratio.
According to this method either information concerning the sensitivity profiles is used to weight individual contributions (MR data) from each receiving coil to an MR overall image in conformity with the arrangement in space or to differentiate between the contributions from different locations in space to individual acquired MR data. However, such methods are not very robust when the actual coil sensitivity profile deviates from the coil sensitivity profile used in the reconstruction, so that inhomogeneities in intensity and aliasing artefacts occur in reconstructed MR images. Notably the SENSE method is very susceptible in this respect.
When receiving coils which do not remain in a fixed location during the entire data acquisition are used for the acquisition of MR data, the assumption of invariant receiving coil sensitivities is incorrect. Notably movements as well as deformations contribute to changing spatial coil sensitivities. One possibility of counteracting such changes of the receiving coil sensitivities during the data acquisition would be the repeated acquisition of calibration data in the case of suspected significant coil movements; such calibration data enable the current coil sensitivity profile to be determined again. However, this approach would require an additional amount of measuring time and also data processing which, depending on the relevant application, is either not desirable or not possible.
For the excitation of the examination zone by means of an excitation coil system which includes at least one excitation coil it is also implicitly assumed that the profiles of the individual coils are invariant in time.
Therefore, it is an object of the invention to provide an MR method and an MR device which are capable of forming MR images of improved image quality, that is, even when use is made of movable and/or flexible receiving coils and/or excitation coils, and which notably avoid the described drawbacks.
This object is achieved in accordance with the invention by means of an MR method as disclosed in claim 1 and an MR method as disclosed in claim 5. For the acquisition of MR data it is proposed to acquire position and orientation information of the receiving coils and to correct the input data for the reconstruction on the basis of the position and orientation information acquired so as to compensate for changes in the position and/or orientation of the receiving coils during the acquisition of the MR data prior to the reconstruction of the MR images. For the excitation of the examination zone it is likewise proposed to acquire position and orientation information on the at least one excitation coil and to correct the excitation signal of the at least one excitation coil on the basis of the position and orientation information acquired so as to compensate changes in the position and/or orientation of the at least one excitation coil. The invention is also implemented by means of an MR device as claimed in claim 6 or 7 as well as by means of a computer program as claimed in claim 12.
The invention is based on the recognition of the fact that for a series of applications it is advantageous to acquire information concerning the position and orientation of the coils in the course of the measurement so as to take this information into account directly for the reconstruction of the MR images, that is, instead of carrying out new calibration measurements before, during or after the acquisition of MR data so as to determine the sensitivity profile or excitation profile of coils again, because such a profile may possibly have changed due to motions during or between acquisitions. Suitable means, notably a position measuring device, are then employed to measure motions and/or deformations of coils. A calibration measurement for determining the sensitivity profile of the at least one receiving coil or the excitation profile of the at least one excitation coil, therefore, is required only once prior to the acquisition of the MR data.
In principle calibration measurements could also be dispensed with completely and theoretical models concerning the field distribution of the coils could be used so as to estimate the relevant sensitivity profile on the basis of the knowledge of the position and the orientation of the coils. Later deviations from these profiles can then be corrected on the basis of the information acquired as regards the position and the orientation.
In preferred embodiments it is arranged that as said input data for the reconstruction on the basis of the measured position and orientation information of the receiving coils the sensitivity profiles of the individual receiving coils as well as single images derived from the acquired MR data of the individual receiving coils or the acquired MR data itself can be corrected in accordance with the invention before the reconstruction of an MR overall image.
Analogously, the excitation profile of an excitation coil as derived from a calibration measurement carried out at the beginning can be corrected on the basis of the information concerning the position and orientation of the excitation coil acquired during the data acquisition. In that case the recalculation of the excitation signal for the individual transmitter coils must be performed directly, that is, the sensitivity profile is estimated on the basis of the acquired position and orientation and therefrom the changed excitation signal is calculated and output directly.
For the correction use can be made of, for example, affine or elastic transformations, or a direct calculation of the changed profile or a combination of the two approaches can be used. The invention thus enables MR images to be obtained which have a significantly higher image quality, that is, even in the case of motions or deformations of receiving coils or excitation coils, and artefacts can be avoided to a high degree.
A position measuring device is preferably used for the acquisition of the position and orientation information. This device may in principle have an arbitrary construction; for example, it may be based on an optical, electromagnetic or microcoil principle. For example, a camera can be mounted on a stand for very accurate determination of the position in space of LEDs provided on the coils. For the position measuring device it is essential that the measuring accuracy of the position measurement is as high as possible, that is, preferably of the order of magnitude of a pixel or less. It is also advantageous when the measurement can be performed continuously. However, it is in principle also possible to use the MR device itself, that is, the excitation and receiving coils of the MR device, for the acquisition of position and orientation information of the coils.
A further embodiment of the MR device in accordance with the invention is provided with flexible coils which are capable of entering a state of stiffness when MR data is to be acquired. In principle various constructions are feasible in this respect, for example, as indicated in the claims 10 and 11. It is essential that the coils, that is, excitation coils as well as receiving coils, may remain flexible and movable for as long as no data is acquired. However, when the acquisition of data commences, the coils are made to enter the state of stiffness so that they can no longer move or be bent at all or to a slight degree only. As a result, the MR images ultimately reconstructed can be additionally enhanced. The melting point of the liquid used in the embodiment as disclosed in claim 11 may be chosen to be such that it is liquid at ambient temperature and solidifies at a temperature which is a few degrees below ambient temperature, so that the coils can be made to enter the state of stiffness by cooling the receptacle during the data acquisition, or that the liquid is solid at ambient temperature and becomes liquid at a temperature a few degrees above ambient temperature, so that for the positioning of the coils the coils can be set to the flexible state by heating the receptacle.
In intermediate spaces between the object to be examined and one or more coils of the receiving coil system or the excitation coil system in a further embodiment of the MR device in accordance with the invention there is provided a material which does not produce an MR signal during the acquisition of the MR data but delivers an MR signal only during a calibration measurement for the determination of the coil sensitivity of a receiving coil or for the determination of the excitation field of an excitation coil. The sensitivity profile or the excitation field acquired by means of the calibration measurement can thus be determined with a higher accuracy, because during the calibration measurement no signal loss is incurred in the intermediate space between the examination object and the coil. Thus, the sensitivity profile or the excitation field is also known in this intermediate space after the calibration measurement. On the other hand, the material present in this intermediate space does not make a contribution to the measured MR data, so that ultimately the MR image will not be falsified thereby. The determination of the sensitivity profile in the xe2x80x9cenvironmentxe2x80x9d of the patient is of interest notably because a subsequent motion may suddenly necessitate estimates for this region in the image reconstruction.